1. Field of the Invention
The invention relates to a power management system for a monitor, a method of power management for a monitor, and a display apparatus with such a power management system.
2. Description of the Related Art
In principle, the determination of eventual variations in time of the video signal may be performed by comparing, in any frame, the value of every pixel with the value of the corresponding pixel in the preceding/following frames. Such an approach is known from Japanese Patent Application No. JP-A-5,344,371. This type of solution may be the most efficient one in terms of certainty of comparison, but, on the other hand, it is the most difficult one to realize in practice in that:
it requires very fast and large memories capable of storing at least one frame;
it requires a sampling signal at the pixel rate for the video input, which is not available in current analog monitors; and
it requires A/D converters operating at very high frequencies, leading to possible errors in the sampling in subsequent frames.
It is, inter alia, an object of the invention to provide power management in a monitor in which variations of a video signal are detected with a less complex circuit which operates at a lower frequency.
To this end, a first aspect of the invention provides a power management system for a monitor having a display screen, the power management system comprising a detector for determining, during successive periods in time, transition numbers indicating a number of transitions in a video signal for a predetermined area of the display screen, a comparator for comparing said transition numbers, and a controller for activating a power down mode of the monitor when at least two of said transition numbers are substantially equal. A second aspect of the invention provides a method of power management for a monitor having a display screen, the power management method comprising the steps of determining, during successive periods in time, transition numbers indicating a number of transitions in a video signal for a predetermined area of the display screen, comparing said transition numbers, and activating a power down mode of the monitor when at least a predetermined number of said transition numbers is substantially equal. A third aspect of the invention provides a display apparatus comprising a power management system as described above.
More particularly, the invention relates to power management in a monitor through the sensing of the steadiness/variation in time of the content of the video signal.
The power management system comprises a detector for determining, during successive periods in time, transition numbers representing a number of transitions in a video signal for a predetermined area of the display screen. Thus, during a period in time of the video information corresponding to part of the video signal which is displayed on the selected area on the display screen, the number of transitions in the video signal are indicated by the transition number. This is repeated in successive periods in time wherein video information is written to the same area of the display screen. In this way, a sequence of transition numbers occurs representing the number of transitions in the video signal for the same area of the display screen in successive periods of time. Two or more transition numbers which are substantially equal in the different time periods, indicate that the video signal in the selected area has the same number of transitions, and thus it is likely that the video signal did not change. Therefore, a comparator compares these transition numbers, and a controller activates a power down mode of the monitor if a sufficient number of transition numbers is equal.
In this way, it is not required to store the values of all the video samples (or pixels) in the selected area for several time periods, and to compare all the corresponding values to determine whether a value of a video sample changed from the one time period to another time period. It suffices to keep track of the number of transitions during each time period. Only a single value per time period needs to be compared. It is possible to compare two or more successive transition numbers to determine whether two or more transition numbers are equal, and if yes, to activate a power down mode of the monitor wherein parts of the monitor or the complete monitor are inactive. It is also possible to continuously compare two successive transition numbers and to keep track of the number of successive transition numbers that are equal. If this number of equal transition numbers surpasses a predetermined value, the power down mode is activated.
A counter counting the number of transitions during the selected time period may generate the transition number which equals the number of transitions occurring during the time period. Alternatively, a pseudo-random generator, also referred to as sequencer, may generate the transition number. Now, the transition number is a pseudo-random number of which the value does not directly provide the number of transitions which occurred during the time period, but which value is indicative for this number of transitions. Such a sequencer may have advantages over a counter.
In an embodiment of the invention characterized in that the predetermined area of the display screen is the whole area covered by a visible part of the video signal, and the successive periods in time are frames of the video signal, each transition number represents the number of transitions in the active part of a complete frame of the video signal. This active part of the frame is displayed as the visible part of the video signal on the screen.
In an embodiment of the invention characterized in that the detector comprises a slicer for supplying a slicer output signal indicating when the video signal crosses a threshold, the slicer compares the video signal with a reference value or level to indicate whether a transition in the video signal occurred. If the video signal is a digital signal, the slicer may compare the samples of the video signal with a reference value. If the video signal is an analog signal, the slicer may compare the video signal with a reference level.
In an embodiment of the invention characterized in that the video signal comprises a first color signal, a second color signal, and a third color signal, the detector comprising a first slicer for receiving the first color signal, and a first pseudo-random generator for receiving an output signal of the first slicer to supply a first sequence of numbers, a second slicer for receiving the second color signal, and a second pseudo-random generator for receiving an output signal of the second slicer to supply a second sequence of numbers, a third slicer for receiving the third color signal, and a third pseudo-random generator for receiving an output signal of the third slicer to supply a third sequence of numbers, the first pseudo-random generator further having an input for receiving an output signal of the third pseudo-random generator such that the first sequence of numbers also depends on the third sequence of numbers, three slicers, and three pseudo-random generators are used, each one to generate the transition number for one of the three color components (also referred to as color signals, usually: the Red, Green and Blue color signals) of the video signal. Each of the slicers compares the color signal sliced with a reference value or level to indicate whether a transition in this color signal occurred. In case of an analog video signal, the slicers may be threshold comparators for converting the analog video levels (for example, in the nominal range 0 . . . 0.7V) in a sequence of digital signals (for example, in the range 0 . . . 5V or 0 . . . 3.3V). Therefore, in any row of the active video, for each of the three colors, a certain number of 0-to-1 or 1-to-0 transitions occur, depending on the picture which is represented on the screen.
A simple counting of these transitions in any frame already provides rough information on the changes of the video content, but this method is unable to detect the eventual changes in the mutual position of the transitions of the three colors among subsequent frames.
To solve this uncertainty, the first step is to introduce, for each color, generators of pseudo-random sequences; these generators behave like counters, but they have the peculiarity that a state has a numerical difference from the previous/following state different than xe2x80x981xe2x80x99; in the counters, the difference between two subsequent states is always xe2x80x981xe2x80x99. This means that any stage inside the sequencer does not show a regular alteration of zeroes and ones (like in a traditional counter), but a random stream of bits. If these pseudo-random generators have a width of 16 bits, each of them can accept 65535 transition before the same sequence in the stream restarts.
Keeping these generators independent of each other, the mutual position of transitions is not yet taken into consideration, and the final state still depends only on the number of the transitions in the three colors.
If, in addition, this random behavior is conditioned by the behavior of another color sequencer, a system is realized in which the terminal state (at the end of any selected time period, for example, a frame) of one or each color sequencer depends both on the number of transitions of its own color, and on the sequence of the conditioning color transitions. In this way, the circuit takes into consideration both the number of transitions of each color and their mutual position for each frame.
Such a known pseudo-random generator generates a pseudo-random output number for each input transition. After initializing (resetting) the pseudo-random generator, the output numbers occur in a fixed order, but are not successive numbers. This means that any stage inside the sequencer does not show a regular alternation of zeroes and ones (like in a traditional counter), but a random stream of bits. Such a sequencer is, for example, known from U.S. Pat. No. 3,976,864 as a signature generator for testing a RAMDAC. This patent discloses several embodiments of signature generators. In a first embodiment, the signature generator is a state machine having an internal state which is a function of the input and its own previous internal state, and having an output which is a function only of the internal state. In another embodiment, the state machine is a multi-element shift register which comprises a series of flip-flops of which the output of one flip-flop drives the input of the next. A feedback is provided by adding output bits to the input.
In an embodiment of the invention characterized in that the detector comprises a pseudo-random generator for receiving the slicer output signal to generate a sequence of pseudo-random numbers, and a screen slicer circuit for receiving a line and a frame synchronization signal to supply a screen slicer signal to the pseudo-random generator, the screen slicer output signal having a different value in different area segments of the display screen, a screen slicer has been added. If only the mutual color transitions are considered, regardless of their spatial position in the frame, situations could easily occur in which variations could not be detected. A typical example is the movement of the mouse pointer on a uniform background. To solve this problem, it is necessary to condition the pseudo-random sequence generators by the information of their spatial position. For this purpose, a fourth pseudo-random generator is added to realize the function of the screen slicer. The fourth pseudo-random generator may generate, in each row, a stream of numbers (for example, 256 bits long), in such a way that each row in a frame has a stream different from that of the other rows. In other words, the screen slicer divides the screen vertically in N slices of P rows each (where P=(Vertical Resolution)/N) and horizontally each row in, for example, M pieces so that P*M=256. With reference to this example, the vertical movement of even one line is detected; a horizontal movement is detected if it is bigger than 1/M of a row. It must be considered that {fraction (1/256 )} of a row is represented by approximately 8 pixels at the maximum resolution, and thus, there is a very high probability of detecting any movement of an icon. In a practical implementation, good results were obtained by dividing the lines in 16 segments.
To complete the system, the final state of the three sequencers is stored and compared with the final state of the previous frame, giving, as a result, a xe2x80x98equalxe2x80x99 or xe2x80x98differentxe2x80x99 signal. The permanence of this signal in the xe2x80x98equalxe2x80x99 state can be elaborated and consequent actions can be taken.
The invention offers the advantage that it is not required to use signals from a PC which supplies the video signal in the monitor""s power management system. An embodiment of the invention is formed by an electronic circuit in the monitor that stores the content of the video signal in numeric form and compares the value with the value of the previous frame. If the values are equal (for two or more successive frames), then a timer is activated that sets the monitor in standby after a selected time period. Otherwise, the timer is reset.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.